The invention relates to fixtures for workpieces, especially for complex workpieces which cannot be readily installed in and properly held by standard work holders of machine tools, such as grinding machines. Typical examples of workpieces which cannot be readily mounted in a grinding machine are portions of compressors and turbines, such as guide vanes, blades and similar parts.
It is well known that special fixtures must be designed to properly hold certain complex workpieces in various types of machine tools. In many instances, the configuration of a workpiece is so complex that the workpiece cannot be properly engaged by the customary work holding means of a machine tool. Moreover, it is often desirable to avoid pronounced clamping engagement between a workpiece and a work holder while a selected portion of the workpiece is being treated by a material removing tool. Still further, many workpieces are configurated in such a way that they do not present relatively large surfaces which can be properly engaged by a work holder. In all such instances, proper treatment of workpieces in a machine tool (such as a milling or grinding machine) invariably necessitates the provision of specially designed fixtures which can be brought into requisite supporting, retaining and locating or orienting engagement with one or more workpieces prior to installation of the fixture in a machine tool. As mentioned above, certain parts (especially blades or vanes) of turbines and compressors are typical examples of such workpieces. For example, if the foot, tip or end piece of a guide vane or blade forming part of a turbine or a compressor must be ground in an automatic grinding machine, such workpieces are normally installed in fixtures each of which contacts the blade or vane proper so that the end piece, foot or tip of the workpiece remains exposed and can be treated by one or more material removing tools. In accordance with a prior proposal which is disclosed in commonly owned German Offenlegungsschrift No. 35 35 646, the fixture consists of or includes a casting which surrounds the blade or vane of a turbine or compressor so that the part or parts which require treatment in a machine tool remain exposed. The material of the fixture is hardened and it completely surrounds a selected portion of or the entire vane or blade but leaves exposed all such parts which must be accessible for proper removal of material from the workpiece in a grinding machine or in another machine tool. This simplifies the mounting of the workpiece in a machine tool because the configuration of the fixture can be readily selected in such a way that it is best suited or most satisfactory for proper installation and retention in the machine tool. When the treatment is completed, the fixture is usually destroyed, for example, by melting or by subjecting it to a mechanical comminuting action so that it falls apart into two or more pieces and permits removal of the confined portion of the treated workpiece. Such types of fixtures are quite satisfactory and are extensively used in many machine tools, especially for proper retention of complex and/or highly sensitive workpieces which should not be directly engaged by the work holder or by the work clamping means of a machine tool.
However, the just described fixtures also exhibit certain serious drawbacks. For example, the material of the fixture is in direct contact with the material of the workpiece. If the consistency of the material of the workpiece and/or the consistency of the material of the fixture is such that the cast and subsequently hardened material of the fixture tends to adhere to the workpiece, actual liberation of the workpiece from its fixture must be followed by lengthy and complex cleaning in order to remove all traces of the material of the fixture before the freshly treated and cleaned workpiece can be installed in a turbine, in a compressor or in a like machine.
Another drawback of the just discussed fixtures is that certain types of workpieces consist of or contain a material which reacts with the molten material of the fixture so that the consistency of the material of the workpiece is changed as a result of direct contact with the material of the fixture. This can involve metallurgical, chemical and/or other changes of the material of the workpieces. The likelihood of undesirable reaction is especially pronounced at the time when the material of the fixture in still hot, i.e., when molten material is being poured into a mold which also contains one or more selected portions of a workpieces.